Water-based coating composition

Abstract

Provided is a water-based coating composition capable of exhibiting high coating film performance even when cured at relatively low temperatures. Disclosed is a water-based coating composition including (A) a hydroxyl group-containing resin and (B) a hardener, wherein at least one type of (A) the hydroxyl group-containing resin is a hydroxyl group-containing acrylic resin particle (A1) having a core-shell structure containing a gradient polymer layer.

1 . An aqueous coating composition comprising (A) a hydroxyl group-containing resin and (B) a curing agent,
 wherein at least one hydroxyl group-containing resin (A) consists of hydroxyl group-containing acrylic resin particles (A1) with a core-shell structure including a gradient polymer layer.   
     
     
         2 . The aqueous coating composition according to  claim 1 , wherein the glass transition temperature of the hydroxyl group-containing acrylic resin particles (A1) with a core-shell structure including a gradient polymer layer is 20° C. or higher. 
     
     
         3 . The aqueous coating composition according to  claim 1 , wherein the glass transition temperature of the shell section of the hydroxyl group-containing acrylic resin particles (A1) with a core-shell structure including a gradient polymer layer is 40° C. or higher. 
     
     
         4 . The aqueous coating composition according to  claim 1 , wherein the proportion of the gradient polymer layer in the hydroxyl group-containing acrylic resin particles (A1) with a core-shell structure including a gradient polymer layer is in the range of 20 to 80 mass % with respect to the total amount of the copolymerizing components in the hydroxyl group-containing acrylic resin particles (A1) with a core-shell structure including a gradient polymer layer. 
     
     
         5 . The aqueous coating composition according to  claim 1 , wherein at least one curing agent (B) is a blocked polyisocyanate compound (B1). 
     
     
         6 . A method of forming a coating film, comprising applying an aqueous coating composition according to  claim 1  onto an article to be coated to form an uncured colored coating film, and then heating and curing the uncured colored coating film at a temperature in the range of 60 to 110° C. 
     
     
         7 . An automobile body coating method comprising a step of forming a first multilayer coating film on an outer plate of an automobile body, and a step of forming a second coating film on an inner plate of an automobile body, wherein the step of forming the first multilayer coating film includes:
 (1a) an outer plate colored base coating film-forming step in which an aqueous coating composition (X) is coated onto an outer plate of the automobile body to form an uncured outer plate colored base coating film (BCo),   (1b) an outer plate clear coating film-forming step in which a clear coating material (Y) is coated onto the uncured outer plate colored base coating film (BCo) to form an uncured outer plate clear coating film (CCo), and   (1c) a baking step in which the uncured outer plate colored base coating film (BCo) and the uncured outer plate clear coating film (CCo) are heated to simultaneously cure the coating films,   the step of forming the second coating film includes:   (2a) an inner plate colored coating film-forming step in which the aqueous coating composition (X) is coated onto an inner plate of the automobile body to form an uncured inner plate colored coating film (PCi), and   (2b) a baking step in which the uncured inner plate colored coating film (PCi) is heated and cured, and   the aqueous coating composition (X) is an aqueous coating composition according to  claim 1 .   
     
     
         8 . The automobile body coating method according to  claim 7 , wherein the heating temperature in the baking step (1c) and the baking step (2b) is in the range of 60 to 110° C. 
     
     
         9 . The automobile body coating method according to  claim 7 , wherein the baking step (1c) and the baking step (2b) are carried out simultaneously.